1. Field of the Invention
The invention relates to a device for the positioning of the root of the blades of a turbojet, for example the fan blades. It relates more precisely to a spacer intended to correctly position the root of the blades in their housing, provided on the periphery of a rotor disk.
2. Discussion of the Background
In a turbojet, the blades, consisting of a root and an airfoil defined by two sides called the pressure face and the suction face respectively, are mounted in groove-shaped housings, called slots, provided especially on the periphery of the rotor disks. These slots may be straight or curved. The blades are mounted with a clearance between their root and the walls of the slots. For the turbojet to operate correctly, the blade roots and the slots of the disk must be in contact over the entire rotation speed range of the engine. The surfaces of contact between the blade roots and the slots are called bearing surfaces. The clearance between the bottom of the slots and the end of the blade roots must be taken up in order to ensure the correct positioning of the blade roots on the bearing surfaces of the disk. In addition, this contact makes it possible to limit the tilting of the blades in their slots and, in certain cases of malfunction, for example in the event of an impact following the ingestion of a foreign body or in the event of the loss of a blade, also to limit the risk of frictional wear or other possible deterioration of the bearing surfaces of the blades or of the disk.
In a known manner, the blades are positioned and kept in place by spacers slid into the slots under each blade root.
In the case of the loss of a blade, the broken blade will impact the adjacent blade, which must be able to tilt about its root in order to release an angle at its tip so that the broken blade slides between the tip of the adjacent blade and the casing situated around the assembly consisting of the rotor disk and the blades. The spacers must therefore, under the effect of the high force resulting from the impact of the broken blade on the adjacent blade, allow the latter to pivot in the slot of the disk. This pivoting must also be controlled in the event of an impact associated with the ingestion of a foreign body by the turbojet, whether or not there is breakage of a blade.
Patent FR 2 841 609 describes a bi-material spacer, consisting of a metal part and an elastomer part. This type of spacer makes it possible to take up the clearance existing between the blade roots and the bearing surfaces of the slots. It also makes it possible to damp the vibrations and allows a slight rotation of the root in the slot in the case of breakage of an adjacent blade, while absorbing part of the impact energy. A first disadvantage of this solution is that installing the spacer under the blade root and dismantling it are difficult because of the coefficient of adhesion of the elastomers. Furthermore, over time, elastomers tend to shrink, which degrades the quality of take-up of the clearance between the blade roots and the bearing surfaces of the slots and increases in the risk of frictional wear of the bearing surfaces of the blades and of the disk.